1. Field of the Invention
Embodiments of the invention generally relate to methods and apparatus for measuring deformation characteristics of a deformable object through changes in, or displacement of, the surface of the object during, over, throughout, and simultaneous with (all hereinafter referred to as ‘during”) a deformation event or deformation time interval (referred to hereinafter as ‘deformation interval’) caused by an inflicted deformation or other perturbation (referred to hereinafter as ‘deformation’) of the surface, and determining elastic and/or viscoelastic properties of the object therefrom. More particularly, embodiments of the invention relate to apparatus and methods that provide the capability to determine biomechanical (non-time dependent) and biodynamic (time dependent), and other characteristics (e.g., intraocular pressure) of a live cornea through measured changes in the topographical characteristics, e.g., displacement of, the surface of the live cornea during a deformation interval caused by an inflicted deformation of the live corneal surface. Most particularly, embodiments of the invention pertain to apparatus and methods that provide the capability to obtain spatially-resolved measurements of displacement, stiffness, corneal elasticity, corneal viscosity, and other biomechanical and biodynamic properties across substantially the entire corneal surface or a more centralized region of a live cornea through changes in the surface topography of the cornea, and pressure measurements, during a deformation interval caused by an inflicted deformation of the live corneal surface.
2. Description of Related Art
The measurement of the surface characteristics or displacement of an object during, over, within, or throughout a deformation of the object's surface can reveal much information about the physical and mechanical properties of the object. If the surface of the object is deformable in response to an applied force, measurement of the changes in characteristics, as well as the amount of displacement of the surface may provide further useful information about the underlying structure of the object and an understanding of how the structure behaves.
There exists numerous organic and inorganic objects having deformable surfaces whose measurement may be of interest in various fields. A particularly interesting, exemplary object is the cornea of a living eye. The widespread interest in understanding the physical, biomechanical, biodynamic, optical, and all other characteristics of the eye is obviously motivated. Over the years, different theories have been presented about the biomechanical and biodynamic properties of the eye, particularly the cornea. Earlier, inaccurate models of the cornea as a solid, rigid structure have more recently given way to newer theories informing a more accurate understanding of the cornea as a layered, dynamic structure that to this day is not completely understood.
Measurement of the biomechanical (elastic) and biodynamic (viscous and viscoelastic) properties of the cornea have previously required a donor (dead) cornea that could be cut into strips and stretched to determine the stress/strain relationship needed to calculate elastic modulus; or an intact donor eye, where the intraocular pressure could be significantly raised in order to stretch the cornea and measure the stress/strain relationship. Alternatively, the viscoelastic properties could be measured by stretching dissected corneal strips and monitoring their changes over time. However, until the advent of the instant embodied invention, it was not known how to measure stress/strain relationships and viscoelastic properties of a live eye (i.e., in-vivo).
An increased but incomplete understanding of the structure of the cornea and its interaction with other components of the eye has been achieved indirectly by measuring various topographical characteristics of the cornea in an unperturbed or undeformed state. These topographical characteristics include corneal curvature and surface elevation with respect to a reference surface, as well as others known in the art. Devices for measuring various topographical characteristics of an object such as a cornea, for example, include topographers, keratographers and keratometers. As known in the art, a topographer is a generic term referring to an apparatus for measuring the topographical characteristics of an object surface. A keratometer is a device that measures corneal curvature only in the central 3 mm of the cornea by measuring the chord length of the virtual image of a target (e.g., ring patterns) imaged by the cornea, and determines curvature based on the index of refraction. A keratographer is a different device that is capable of making measurements beyond the central 3 mm region of the cornea based upon reflection from the anterior corneal surface rather than imaging through the cornea. Thus different devices use different measuring principles to determine various topographical characteristics of the cornea. For example, some devices use Placido-based reflective image analysis. Placido-based devices can measure curvature parameters of the cornea but typically lack the capability to directly measure surface elevation. The Orbscan® anterior segment analyzer (Bausch & Lomb Incorporated), on the other hand, is a topographical characteristic measuring device that utilizes a scanning optical slit. Device software provides for direct measurement of surface elevation and corneal thickness as well as surface curvature. Another commercial device developed by Par Technology Corporation is known as the PAR CTS™ Corneal Topography System (PAR). The PAR CTS system utilizes a raster photography method. The PAR CTS operates by projecting a known grid image onto the anterior corneal surface that is viewed by a camera from an offset axis. Other topographical characteristic measuring techniques may include Scheimpflug Tomography, confocal microscopy, optical coherence tomography, ultrasound, optical interferometry, and others, all of which are well known in the art.
While the measurement of various topographical characteristics of the cornea provide a wealth of information about vision and the effects of corneal shape on visual performance, corneal topography by itself cannot reveal the biodynamic and biomechanical properties of the cornea necessary for a thorough understanding of its structure and functional operation; thus it is necessary to know something about the elastic and viscoelastic properties of the cornea. For example, Liu and Roberts, J Cataract Refract Surg 2005; 31:146-155, first recognized the impact that biomechanical and biodynamic properties of the cornea have on intraocular pressure (IOP).
An illustrative apparatus in which biomechanical properties have been determined to influence the measurement process, unknown to the user and responsible for errors in measurement, is a tonometer. Tonometers, which are devices for determining intraocular pressure (IOP), were originally developed as contact-type instruments, meaning that a portion of the instrument is brought into contact with the corneal surface during the IOP measurement procedure. A well known instrument of this type is the Goldmann applanation tonometer (GAT), originally developed in the 1950s. The GAT measures the force required to flatten (“applanate”) a known area of the cornea, from which IOP can be determined if assumptions are made about the thickness and properties of the cornea. The GAT is used today as a standard against which other types of tonometers are compared to assess measurement accuracy.
It has been reported in the literature that corneal thickness generates errors in GAT measurements. However, it has also been theoretically predicted that biomechanical properties (e.g., elastic modulus) have a greater influence on measurement error than corneal thickness. Id.
Patient discomfort caused by contact tonometers such as the GAT led to the development of “non-contact” tonometers, which operate by directing an air pulse generated by a pump mechanism through a discharge tube aimed at the cornea to cause applanation. As the cornea is deformed by the fluid pulse, an optoelectronic system monitors the cornea by detecting corneally-reflected light from a beam obliquely incident upon the cornea. A peak detector signal occurs at the moment of applanation when the reflecting surface of the cornea is flat. During a non-contact IOP measurement, and depending on the characteristics of the piston, the cornea is actually deformed from its original convex state through a first state of applanation to a slightly concave state and then through a second state of applanation to convexity as the air pulse decays.
A method for measuring air puff force and determining IOP, and a non-contact tonometer, are disclosed in U.S. Pat. Nos. 6,419,631 and 6,875,175, the disclosures of which are hereby incorporated by reference in their entireties to the fullest extent allowed by applicable laws and rules. This technology is commercially known as the Reichert (Depew, N.Y.) Ocular Response Analyzer™. According to posted information accessible at http://ocularresponse.reichertoi.com, the Reichert Ocular Response Analyzer utilizes a dynamic bidirectional applanation process to measure a cornea tissue property called corneal hysteresis. The term corneal hysteresis refers to the difference in pressure values of the air pulse at the inward moving applanation point and the outward moving applanation point during a measurement interval (inward moving refers to an initial convex corneal shape moving to a flattened condition, while the outward applanation point refers to the maximum air pulse concave corneal surface moving towards the applanation point on its return to a normal convex surface shape as the air pressure decays). Since corneal hysteresis appears to be a repeatable measurement, it may provide a metric that is useful for identifying and categorizing various conditions of the cornea. For example, measurement of corneal hysteresis is alleged to aid in identifying and classifying conditions such as corneal ectasia and Fuch's Dystrophy, and as helping in the diagnosis and management of glaucoma. Differences in hysteresis measurements for different corneal conditions may better inform about the biomechanical and biodynamic properties of the cornea. Because corneal hysteresis measurement is credited for presenting one characterization of the cornea's biomechanical state, it is believed to have additional potential uses in screening refractive surgery candidates as well as predicting and controlling surgical outcomes. However, none of these predictions and controlling of surgical outcomes have been validated in scientific studies. In addition, corneal hysteresis is not a measurement of elastic modulus or corneal stiffness. It has been reported in the literature that hysteresis does not correlate to elastic modulus by its very nature. The same value of hysteresis is associated with multiple combinations of viscosity and elasticity. For example, low hysteresis is associated with both a soft cornea in the case of keratoconus, and a stiff cornea in the case of an older eye or an eye with higher pressure. Hysteresis has been shown to decrease with a subject's age, where it is well known in the art that corneas stiffen with age. Therefore, corneal hysteresis is not the answer to providing a quantitative measurement of corneal biomechanical and/or biodynamic properties.
There is thus a clear need to be able to measure corneal biomechanical and biodynamic properties in-vivo, which to the inventor's knowledge has never been accomplished on a live eye.
In view of the shortcomings of the above described techniques, capabilities, and apparatus for measuring corneal topographical characteristics, hysteresis, and other known parameters in a serial manner, the inventor has recognized that additional benefits could be obtained by developing an apparatus and method that involve integration of the different concepts and are capable of real-time, in-vivo measurements of corneal biomechanical properties. The inventor has further recognized the need for new and improved methods and apparatus that are capable of measuring and quantifying biomechanical properties of the cornea in a manner not previously demonstrated or accomplished, such as making and obtaining spatially-resolved, in-vivo corneal deformation characteristic measurements over a specified region of the cornea (e.g., central cornea or corneal-scleral region), resulting in a better understanding of corneal biomechanics and biodynamics.
These and other advantages and benefits are achieved by the invention, which will be described in detail below and with reference to the drawings.